The Question:

Does an acre have to have a specific shape? In The
Wall by Pink Floyd, the teacher says the area of an acre
is a rectangle that is one furlong in length and one chain in width.
How long is a chain and how wide is a furlong?

The Answer:

Not as such. The original definition of an acre was, indeed, a
furlong by a chain, a definition used by surveyors to measure plots.
However, the area one derives from that measurement is an acre, even
if it's contained within a different shape.

1 furlong is equal to 10 chains, or 660 feet. If you do the
math, this means an acre is 10 square chains, or 43,560 square feet.
An oval-shaped 43,560 square-foot plot will still be 1 acre in
size.